1. Field of the Invention
This invention relates to piezoelectric composites, and more particularly to piezoelectric composites for high-frequency ultrasound applications and methods of manufacturing such composites.
2. Background Art
Typically, high quality medical imaging uses ultrasonic transducers or transducer arrays that posses the properties of good sensitivity and wide frequency bandwidth. Conventional transducers utilizing monolithic piezoelectric material such as, for example, lead zirconate titanate (“PZT”), typically exhibit a large acoustic impedance mismatch between the transducer and the medium under test, such as, for example, water, human tissue, and the like. To overcome this problem, piezoelectric composites that are made of individual small piezoelectric elements, which can be surrounded and isolated by a polymer matrix, such as, for example, epoxy, have been proposed and implemented at low frequencies. These small piezoelectric elements play an increasingly important role in the development of ultrasonic transducers for medical imaging. One commonly used structure of piezoelectric composite consists of small rectangular or square pillars of PZT that are embedded in a host matrix of polymer material. In one example, the height of the pillars is normally about one half of one wavelength at the operating frequency if the backing material is lower in acoustic impedance.
Unfortunately, developing a high-frequency (>15 Hz) ultrasound transducer is also very challenging due to the extremely small pillar dimensions required in order to avoid significant lateral resonances in the piezoelectric composite. Conventionally, the design of piezo-composites is limited by the blade size limit of micro-dicing saws or other conventional apparatuses that are used to cut the bulk piezoelectric into composite pillars. It is very difficult using conventional dice and fill techniques to sufficiently reduce the size/spacing of the composite pillars enough to push the lateral resonances outside the operating bandwidth of a transponder that is configured to operate at high frequencies. For example, to push the first “lamb mode” frequency to about 80 MHz, while still maintaining a volume fraction of piezoelectric above 25%, a kerf width of approximately 6 μm is required (assuming a typical piezoelectric and epoxy filler). What is needed is a high-frequency ultrasound transducer that operatively suppresses these lateral modes within the piezoelectric composite.
In a further aspect, a lens is typically used to passively focus high-frequency ultrasound transducers. Developing a suitable acoustic lens, however, can be very challenging because the lens materials commonly used for lower frequency transducers are far too attenuating at frequencies at higher frequencies. Alternatively, the need for an acoustic lens can be avoided by geometrically curving the transducer. This can be accomplished by using a flexible piezo-composite material as the transducer substrate.